In U.S. Pat. No. 4,930,365, whose disclosure is hereby incorporated by reference, there is described a two axis rate gyroscope of the kind comprising an electric motor driven inertia rotor that is distinct from the motor rotor, which inertia rotor is keyed on a portion of the electric motor shaft that comprises a flexible part constituting a flexure shaft adapted to flex upon precession of the inertia rotor in consequence of an angular velocity input around either of two axes normal to each other and both normal to the rotor spin axis; the shaft further comprising an end portion extending beyond the inertia rotor in the direction away from the electric motor constituting a deflector; electro-optical measuring means being provided in association with said deflector for detecting any deflection thereof.
The present invention is based on the suprising realisation that precession of the inertia rotor in a gymballess, two axis rate gyroscope of the kind specified occurs also upon linear acceleration and that the resulting deflection of said deflector can be correlated to the linear acceleration in two axes perpendicular to the shaft provided that the flexure shaft inertia rotor assembly has two degrees of freedom with two different flexure coefficients in two mutually perpendicular directions both normal to the shaft. These directions will be referred to herein as "axis 2" and "axis 3".
It has furthermore been found in accordance with the present invention that when in a gyroscope of the kind specified the inertia rotor is replaced by a concentrated mass body, i.e. a body of low volume, linked to the motor shaft by a flexure zone permitting a transversal parallel shift of said concentrated mass body, there results an apparatus in which linear acceleration brings about such transversal parallel shift.
Based on these findings the invention provides a two-axis accelerometer comprising an electric motor driven rotating body that is distinct from the motor rotor and linked to the latter via a flexure zone to form a rotating body/flexure zone combination, which combination is selected from the group consisting of type (i) and type (ii) combinations, said type (i) combination comprising a concentrated mass in combination with a flexure zone of a kind that permits the rotating body to perform a transversal parallel shift, and said type (ii) combination comprising an inertia rotor in combination with an extension of the electric motor shaft that embodies a flexible portion constituting said flexure zone; which type (i) and type (ii) rotating combinations each have two degrees of flexing freedom with two different flexure coefficients in two mutually perpendicular directions both normal to the electric motor rotor shaft; there being provided a deflector shaft coaxial with the rotor shaft and extending beyond the rotating body in a direction away from the electric motor, and in association with said deflector shaft electro-optical measuring means and processor means for measuring linear acceleration components in said two mutually perpendicular directions.
When the rotating body has a concentrated mass, the flexure zone that permits transversal parallel shift of the body may, for example, be in the form of a plurality of peripherally arranged flexure rods.
In the following, the assembly of a flexure zone, a rotating body, a deflector shaft, electro-optical measuring means and processor means will occasionally be referred to as "deflector assembly". A deflector assembly with a rotating body/flexure zone combination of type (i) above will be referred to as "deflector assembly of the first kind" and a rotating body/flexure zone combination of type (ii) above will be referred to as "deflector assembly of the second kind".
Similar as in U.S. Pat. No. 4,930,365 the terms "axisymmetric" and "asymmetric" are used herein in relation to the geometry of the flexure zone and the inertia rotor in a gyroscope according to the invention. The term "axisymmetric" is used in its conventional meaning to denote a body that is symmetrical all around a central axis. The term "asymmetric" is used in respect of bodies that are not axisymmetric but which may still be symmetric with respect to one or more planes extending through a central axis.
Thus, an accelerometer according to the invention may exist in either of two modes. By one mode, to be referred to as "accelerometer of the first kind", the apparatus has a deflector assembly of the first kind. By another mode, to be referred to as "accelerometer of the second kind", the apparatus has a deflector assembly of the second kind.
In its simplest form the apparatus according to the invention is exclusively an accelerometer and in such an embodiment one set of electro-optical measuring means in association with one set of processor means is sufficient. Where such an apparatus has a deflector assembly of the first kind, the performance of the deflector can be expressed by the following algorithmic expression: EQU x.sub.2 =-a.sub.2 .alpha..sub.1 +.alpha..sub.2 (a.sub.2 cos 2nt+a.sub.3 sin 2nt) I
where:
a.sub.2 and a.sub.3 are the acceleration components at the two mutually perpendicular axes 2 and 3;
.alpha..sub.1 and .alpha..sub.2 are constants depending on the flexure coefficients in these two directions, the rotor mass and the speed of rotation;
n is the motor speed; and
x.sub.2 is the deflection component in axis 2, and the processor means are designed to resolve this algorithmic expression.
Where such an apparatus has a deflector assembly of the second kind, an asymmetric inertia rotor having three different principle moments of inertia is used in association with an asymmetric flexure shaft having two angular flexure coefficients in two perpendicular directions. Due to acceleration in a plane perpendicular to the motor shaft the rotor performs angular movements composed of constant angular bending and bending vibration at frequency twice the rotating speed. The performance of the deflector can in this case be expressed by the following algorithmic expressions: EQU .theta..sub.2 =-.alpha..sub.1 a.sub.2 +.alpha..sub.2 a.sub.2 cos (2nt)-.alpha..sub.2 a.sub.3 sin (2nt) II
where
a.sub.2 and a.sub.3 are the acceleration components at the perpendicular axes 2 and 3;
.alpha..sub.1, .alpha..sub.2 are constants depending on the flexure coefficients in these two directions, the inertia properties of the inertia rotor, the motor speed and the position of the centre of the rotor mass relative to the flexure zone;
n is the rotor speed; and
.theta..sub.2 is the angular deflection of the inertia rotor in axis 2, and the processor means are designed to resolve this algorithmic expression.
In the above algorithm the angular deflection .theta..sub.2 will in addition to the acceleration be also influenced by applied angular velocity. However, by appropriate selection of the parameters (i.e. the inertia and flexure properties as well as the speed of rotation) it is possible to cancel the influence of the angular velocity and in consequence, by measuring .theta..sub.2 it is possible to determine both a.sub.2 and a.sub.3.
The above algorithmic expressions I and II each have a DC component resulting from the expression .alpha., a and an AC component embodying the expressions cos 2nt and sin 2nt.
An accelerometer according to the invention may be designed as a multisensor adapted to measure at one and the same time both linear acceleration components and angular velocities in axes 2 and 3. In accordance with one embodiment of such a multisensor the apparatus is fitted with a first deflector assembly at one end dedicated to measuring linear acceleration, and with a second deflector assembly at the opposite end dedicated to measuring angular velocities.
In said first deflector assembly dedicated to measuring linear acceleration the accelerometer assembly may be either of the first kind whose performance is governed by the above algorithmic expression I, or of the second kind whose performance is governed by the above algorithmic expression II.
In accordance with another embodiment of a multisensor according to the invention there is provided one single deflector assembly with one single inertia rotor, designed for measuring both the linear acceleration components and the angular rate in said axes 2 and 3.
By one modification of such an embodiment the inertia rotor is asymmetric and the flexure shaft is symmetric, and in this case the performance is governed by the following algorithmic expressions: EQU .theta..sub.2 =-.alpha.1.omega..sub.3 +.alpha..sub.2 a.sub.2 +.alpha..sub.3 .omega..sub.3 cos (2nt) -.alpha..sub.3 .omega..sub.2 sin (2nt) EQU .theta..sub.3 =.alpha..sub.1 .omega..sub.2 -.alpha..sub.2 a.sub.3 +.alpha..sub.3 .omega..sub.2 cos (2nt) -.alpha..sub.3 .omega..sub.3 sin (2nt) III
where:
a.sub.2, a.sub.3, n and .theta..sub.2, are as defined in relation to the algorithmic expressions II, .theta..sub.3 is the angular deflection of the inertia rotor in axis 3 and .omega..sub.2 and .omega..sub.3 are the angular velocities in axes 2 and 3 and the processor means are designed to resolve these algorithmic expressions. From the AC components of the above algorithmic expression III the signal processing unit yields the angular rate component .omega..sub.2, .omega..sub.3 and from the DC components the signal processing unit yields the linear acceleration components a.sub.2 and a.sub.3.
By another modification of a multisensor according to the invention with one single deflector assembly, both the inertia rotor and flexure shaft are asymmetric, the performance is governed by the following algorithmic expressions: EQU .theta..sub.2 =-.alpha..sub.1 .omega..sub.3 +.alpha..sub.2 a.sub.2 cos (2nt)-.alpha..sub.3 a.sub.3 sin (2nt) EQU .theta..sub.3 =.alpha..sub.1 .omega..sub.2 +.alpha..sub.2 a.sub.3 cos (2nt)+.alpha..sub.3 a.sub.2 sin (2nt) IV
where .alpha..sub.1, .alpha..sub.2, .alpha..sub.3, a.sub.2, a.sub.3, .omega..sub.2, .omega..sub.3, n and .theta..sub.2, .theta..sub.3 are the same as before and the processor means are designed to resolve these algorithmic expressions.
In this modification, similar as in the modification that operates on the basis of the algorithmic expressions III, the vibrational movements at 2 n frequency (AC-components) are separated from the quasistatic components (DC). However, as distinct from the previous modification, in this modification the signal processing means yields the linear acceleration components a.sub.2 and a.sub.3 from reading the AC-components, while the angular rate .omega..sub.2 and .omega..sub.3 are obtained from the DC-components.
By yet another modification of a multisensor according to the invention with a single deflector assembly in which the inertia rotor is symmetric and the flexure shaft is asymmetric, and the performance of the deflector is governed by the following algorithmic expression: EQU .theta..sub.2 =-.alpha..sub.1 .omega..sub.3 +.alpha..sub.2 a.sub.2 +(.alpha..sub.3 .omega..sub.3 +.alpha..sub.4 a.sub.2) cos (2nt)-(.alpha..sub.3 .omega..sub.2 +.alpha..sub.4 a.sub.3) sin (2nt) EQU .theta..sub.3 =.alpha..sub.1 .omega..sub.2 -.alpha..sub.2 a.sub.3 +(.alpha..sub.3 .omega..sub.2 +.alpha..sub.4 a.sub.3) cos (2nt)+(.alpha..sub.3 .omega..sub.3 +.alpha..sub.4 a.sub.2) sin (2nt)V
where:
.alpha..sub.1, .alpha..sub.2, .alpha..sub.3 and .alpha..sub.4 are constants depending on the rotary inertia properties, the flexure shaft constants and the position of the rotor center of mass and a.sub.2, a.sub.3, .omega..sub.2, .omega..sub.3, n and .theta..sub.2, .theta..sub.3 are as before. In principle the operation in this case is similar to the modification governed by the algorithmic expressions IV above, the processor means being designed to resolve the algorithmic expressions V.
In accelerometers and multisensors according to the invention all signals are measured with a non-rotating and thus non-abrasive measuring system. Against this, known multisensors, e.g. those manufactured by Plessey Electronic Systems Corporation (previously Singer Kearfott), the signals are measured by a rotary system which comprises piezoelectric beams attached to a rotor and rotating with it. The piezoelectric beams yield electric signal due to an input angular rate or linear acceleration. The electric signals are transferred through slip-rings to a signal processing unit and the system is intrinsically abrasive.